1. Field of the Invention
The present invention relates to surface-mounted electronic components, that is, components having their contact pads supporting conductive protrusions intended to be directly bonded to bonding pads of a support such as a printed circuit board. The present invention more specifically relates to surface-mounted components capable of withstanding shocks and/or temperature variations.
2. Discussion of the Related Art
In surface mounts, to minimize the bulk, bare chips (package-free) having their surfaces only covered with a thin insulating layer, for example, made of silicon oxide, are often used.
FIG. 1A is a cross-section view schematically showing a surface-mountable chip 1 capable of being mounted directly onto the surface of a printed circuit board 3. Chip 1 comprises, on its front surface, metal balls 5 bonded to contact pads (not shown in the drawing) of the chip. Chip 1 is a bare chip only coated with a stack of a thin insulating layer 7 on its front surface (the front surface may also be covered with a stack of several insulating and conductive layers in the case where the balls are connected to the chip via conductive tracks). On its front surface, printed circuit board 3 comprises bonding pads 9. On assembly, balls 5 of the chip bear against bonding pads 9 of the printed circuit. Balls 5 are then bonded to pads 9. The bonding can be, for example, by welding or by any other known bonding technique.
FIG. 1B is a view of the front surface of chip 1, in an example where the chip comprises sixteen contact balls uniformly distributed in a square matrix over the entire chip surface.
In surface mounts, the size of the balls limits the number of electric connections which may be formed between the chip and the printed circuit. As an example, on a square chip having a 4-mm side length, a matrix having at most 8×8 balls with a diameter from 0.25 mm to 0.35 mm and a step from 0.4 mm to 0.5 mm (from center to center) between two neighboring balls is generally provided.
To increase the number of possible connections, it has been provided to form, around the semiconductor chip, a peripheral resin extension, substantially of the same thickness as the chip, this extension being capable of supporting additional contact balls. The resin extension comprises contact pads and conductive tracks at its surface to connect the additional balls to areas of the chip to be contacted.
FIGS. 2A to 2C are cross-section views very schematically showing steps of the forming of thin semiconductor chips provided with peripheral resin extensions.
FIG. 2A illustrates a step during which semiconductor chips 21, previously manufactured and diced according to usual methods, are positioned on a support plate 23. It should be noted that chips 21 will have previously been tested according to usual methods, for example, probe tested, directly on the silicon wafer before chips 21 are diced. Thus, only valid chips 21 are positioned on plate 23, which provides a good manufacturing yield for components provided with peripheral resin extensions. Chips 21 are arranged on plate 23 so that the rear surface of chips 21 remains free. The front surface of chips 21 is for example stuck to support plate 23 via a double coated adhesive. Further, a free space 25 is provided between neighboring chips 21.
FIG. 2B illustrates a step during which a resin deposition is performed on plate 23, so that resin 27 fills free spaces 25 (2A) between chips 21. In practice, to begin with, the thickness of the formed resin layer 27 is greater than the thickness of chips 21. In a subsequent step, resin layer 27 is thinned down to have the same thickness as chips 21. Thus, in top view (not shown), each chip 21 is surrounded with a resin frame 27 solid with the chip, the thickness of resin 27 substantially corresponding to the thickness of chips 21, for example, on the order of from 250 μm to 450 μm.
FIG. 2C illustrates the positioning of contact balls 29 on chips 21 and on the corresponding resin extension 27. In an intermediary step, not shown, the wafer formed by chips 21 and by resin extensions 27 is separated from support plate 23 and may be flipped. Before positioning/bonding balls 29, a stack of insulating and conductive layers capable of forming connections and pads at the surface will have been deposited on the chip and the resin extensions, the connections connecting the pads to contacts on the chip.
In a final step, the extended components are diced.
FIG. 3 is a cross-section view showing in more detailed fashion a component 31 formed according to the method described in relation with FIGS. 2A to 2C. As an example, base semiconductor chip 21 is a square chip with a 4-mm side length, and peripheral resin extensions 27 form, around chip 21, a frame with square sides having a 2-mm width. Thus, chip 21 and resin extension 27 altogether form a square having an 8-mm side length, on which a matrix of 16×16 balls having a diameter from 0.25 mm to 0.35 mm, with a step from 0.4 mm to 0.5 mm between two neighboring balls (from center to center), may be provided.
A problem often encountered in surface mounts is the brittleness of the contact balls. In case of a shock, balls may crack, which will result in malfunctions of the mount. To overcome this disadvantage, it has been provided to partially bury the balls in a protective resin layer formed on the front surface of the chip and having a thickness ranging between 40 and 70% of the ball height. It should be noted that balls 29 are slightly crushed when welded to the connection pads. Thus, balls having a diameter on the order of from 0.25 mm to 0.35 mm (horizontally) will conventionally have a final height on the order of from 0.2 mm to 0.25 mm.
FIG. 4 is a cross-section view showing a component 41 comprising the elements of component 31 of FIG. 3, and further comprising a resin layer 43 for protecting balls 29. Layer 43 is for example formed by resin injection and/or compression on the front surface of the chip, and extends up to approximately 60% of the ball height. In the forming of layer 43, a mold capable of avoiding any resin deposition on the top caps of the balls is generally used, to be able to subsequently form electric contacts on these caps. It should be noted that protective resins usually used are hard and rigid at normal operating temperature of the component. In order to form layer 43, the resin is heated so as to become temporarily malleable.
Resin layer 43 enables to improve the resistance to shocks of contact balls 29. However, the balls remain likely to crack. In particular, balls 29 are embrittled by the mechanical stress which is exerted in a surface mount during temperature variations. Indeed, such a mount comprises different materials having different thermal expansion coefficients (resin 27 of the chip extensions, resin of layer 43, silicon of chip 21, metal of balls 29, printed circuit board support material, etc.). Thus, in temperature variations, the balls are submitted to a significant mechanical stress.
Further, due to the thermal expansion coefficient difference between the substrate of chip 21 and protective resin layer 43, a warpage of the surface mount component is likely to occur. Indeed, when temperature variations occur, the substrate of chip 21 and the resin layer 43 tend to expand or contract by different amounts. The resin 43 and the substrate of chip 21 being rigid materials, with little elasticity or compressibility, this leads to the stack becoming warped.
To minimize this warpage, it has been provided to form, on the rear surface of the chip, a resin layer substantially identical to protection layer 43 formed on the front surface. Thus, in temperature variations, the stress exerted on the chip by layer 43 is compensated by the substantially opposite stress exerted by the rear surface resin layer. However, such a solution requires forming an additional resin layer, which complicates the component manufacturing.